1. Field of the Invention
The present invention relates to a magnetic object motion sensor including a pair of magnetoelectric transducer elements for detecting for example the rotation of a cam shaft of an internal combustion engine, and more particularly, to a magnetic object motion sensor which can output a high-precision pulse signal which precisely corresponds to the motion of the object to be detected, and which can be produced with a greater tolerance in structure and production and thus at low cost.
2. Description of the Related Arts
FIG. 12 is a cross-sectional view illustrating a conventional magnetic object motion sensor as disclosed in Japanese Patent Laid-Open Nos.7-198736 and 6-273437. In this specific example, the magnetic object motion sensor is disposed opposite to a rotating magnetic object so as to serve as a rotation detector for detecting the rotation of the rotating magnetic object. FIG. 13 is a side view illustrating the position of the rotation sensor relative to the position of the rotating magnetic object shown in FIG. 12.
As shown in FIGS. 12 and 13, a moving magnetic object serving as a rotating magnetic object 1 has a plurality of magnetic material protrusions 1a disposed at equal intervals along its periphery.
A pair of magnetoelectric transducer elements, such as Hall devices, 2a and 2b are disposed a predetermined distance D apart from each other in the direction, denoted by xe2x80x9cAxe2x80x9d, of the motion of the rotating magnetic object 1 so that the magnetoelectric transducer elements 2a and 2b face the magnetic material protrusions 1a disposed along the periphery of the rotating magnetic object 1. Each magnetoelectric transducer element 2a and 2b detects a change in magnetic field caused by the motion of the magnetic material protrusions 1a, and outputs an electric signal (which will be described in detail later) corresponding to the change in the magnetic field.
Both magnetoelectric transducer elements 2a and 2b are included in an integral fashion in a sensor IC 3, which also includes other various circuit elements (which will be described later).
A permanent magnet 4 magnetized in the direction denoted by the arrow B (refer to FIG. 13) is disposed adjacent to the sensor IC 3 so that a bias magnetic field is supplied to each magnetoelectric transducer element 2a and 2b. 
The rotation sensor is constructed with the sensor IC 3 including the pair of magnetoelectric transducer elements 2a and 2b and the permanent magnet 4 for generating the bias magnetic field. The rotation sensor is held in an integral fashion by a holding member 5 so that the rotation sensor is maintained at the specific position described below relative to the rotating magnetic object 1.
The above components are disposed so that the central axis of the sensor IC 3 and the permanent magnet 4 passing between the center of the magnetoelectric transducer elements 2a and 2b is coincident with the central axis C in a radial direction of the rotating magnetic object 1. The side face of the permanent magnet 4 facing the rotating magnetic object 1 is parallel to the line from one element to the other of the two magnetoelectric transducer elements 2a and 2b and also parallel to the tangential direction E of rotation movement of the rotating magnetic object 1.
Therefore, the magnetization direction of the permanent magnet 4 is coincident with the direction of the central axis C.
FIG. 14 is a circuit diagram illustrating a typical circuit configuration of the sensor IC 3.
As shown in FIG. 14, the sensor IC 3 has a power supply terminal Vcc via which a power supply voltage is supplied from an external circuit (not shown), a ground terminal GND connected to ground, and an output terminal Vout via which a pulse signal P is output.
The sensor IC 3 also includes: a pair of amplifiers 6a and 6b for separately amplifying electric signals Fa and Fb output by the pair of magnetoelectric transducer elements 2a and 2b; a differential amplifier 7 for amplifying in a differential fashion the electric signals Ga and Gb output by the respective amplifiers 6a and 6b; a waveform shaping circuit 8 for converting the differential signal H output by the differential amplifier 7 into a pulse signal J corresponding to the edges of the magnetic material protrusions 1a; and a transistor 9 with a grounded emitter for,inverting the pulse signal J and outputting the resulting signal as a pulse signal P.
The magnetoelectric transducer elements 2a and 2b, the amplifiers 6a and 6b, the differential amplifier 7, and the waveform shaping circuit 8 are connected to the power supply terminal Vcc and also to the ground terminal GND so that electric power is supplied to these circuit elements.
The pulse signal P output from the transistor 9 via the output terminal Vout is further converted by an external circuit to a pulse signal having a final form.
The operation of the conventional magnetic object motion sensor shown in FIGS. 12 to 14 is described below with reference to the waveform diagrams of FIGS. 15 and 16.
FIGS. 15 and 16 illustrate the relationships among the magnetic material protrusions 1a of the rotating magnetic object 1, the differential signal H input to the waveform shaping circuit 8, and the pulse signal J shaped by the waveform shaping circuit, wherein FIG. 16 illustrates those relationships for the case in which the rotating magnetic object 1 rotates at a low speed.
In FIGS. 15 and 16, TH denotes a threshold level at which the pulse signal J is switched from a high level (H-level) to a low level (L-level), TL denotes a threshold level at which the pulse signal J is switched from a low level to a high level, and xcex94T denotes the hysteresis or the difference between the above threshold levels TH and TL.
As shown in FIG. 16, the circuit characteristic of the sensor IC 3 has the feature that undershoot Ho and overshoot occur in the differential signal H in particular when the rotating magnetic object 1 rotates at a low speed.
When the rotating magnetic object 1 rotates in the direction indicated by the arrow A, the pair of magnetoelectric transducer elements 2a and 2b detects the change in the magnetic field caused by the motion of the magnetic material protrusions 1a of the rotating magnetic object 1, and outputs electric signals Fa and Fb whose voltage changes in response to the change in the magnetic field.
The electric signals Fa and Fb are amplified by the amplifiers 6a and 6b and output as electric signals Ga and Gb, which are further amplified in a differential fashion by the differential amplifier 7 and output as a differential signal H. The differential signal H is then applied to the waveform shaping circuit 8 constructed with for example a Schmitt trigger circuit.
The waveform shaping circuit 8 compares the differential signal H with the threshold levels TH and TL, and converts the differential signal H to a pulse signal J corresponding to the edges of the magnetic material protrusions 1a. 
More specifically, when the differential signal H has become higher than the threshold TH, the pulse signal J is made to fall down to a low level (L level). On the other hand, when the differential signal H has become lower than the threshold TL, the pulse signal J is made to rise up to a high level (H level).
The pulse signal J is then inverted by the transistor 9 into a pulse signal P which is then supplied via the output terminal Vout to an external computer unit or a similar device so as to detect the rotation speed or the rotation angle of the rotating magnetic object 1.
However, as can be seen from FIG. 15, the threshold levels TH and TL associated with the waveform shaping circuit 8 are located in the range in which the differential signal H changes rather gradually. As a result, a small change in level of the differential signal H results in a shift of the pulse signal J in a xcex4 direction, which causes degradation in the detection accuracy of the rotation of the rotating magnetic object 1. Therefore, it is required that the structural accuracy such as the attachment accuracy of the sensor IC 3 should be extremely high. This results in an increase in cost.
Furthermore, when the rotating magnetic object 1 rotates at a low speed, undershoot Hu or overshoot Ho often occurs in the differential signal H as shown in FIG. 16. If such undershoot or overshoot occurs, the waveform shaping circuit 8 responds to the undershoot Hu or the overshoot Ho, and thus the pulse signal J greatly deviates from the normal pulse position (denoted by the alternate long and short dash line in FIG. 16), which results in degradation in rotation detection accuracy.
In the conventional magnetic object motion sensor, as described above, since the waveform shaping is performed in the range in which the differential signal H changes gradually, it is difficult to generate a pulse signal J which precisely corresponds to the edges of the magnetic material protrusions 1a. Although a high-precision pulse signal can be achieved if a high-precision manufacturing process is employed, such process is very expensive.
Furthermore, when the moving magnetic object rotates at a low speed, the waveform shaping circuit incorrectly responds to undershoot or overshoot in the differential signal H, and it becomes impossible to output a pulse signal J which precisely corresponds to the edges of the magnetic material protrusions 1a. 
It is a general object of the present invention to solve the above problems. More specifically, it is an object of the present invention to provide a high-precision magnetic object motion sensor which allows a greater production tolerance and a greater dimensional tolerance, and which can therefore be manufactured at low cost.
According to an aspect of the present invention, there is provided a magnetic object motion sensor comprising: a pair of magnetoelectric transducer elements disposed opposite a magnetic material protrusion of a moving magnetic object; a magnet for generating bias magnetic flux toward the pair of magnetoelectric transducer elements; unbalanced bias producing means for producing imbalance in the magnetic field sensitivity between the pair of magnetoelectric transducer elements; a differential amplifier for amplifying in a differential fashion the electric signals output by the pair of magnetoelectric transducer elements; and a waveform shaping circuit for converting the differential signal output by the differential amplifier to a pulse signal corresponding to the edges of the magnetic material protrusion; the pair of magnetoelectric transducer elements being disposed in such a manner that one element is a predetermined distance apart from the other in the direction of motion of the moving magnetic object so that the pair of magnetoelectric transducer elements detect a change in magnetic flux caused by the motion of the magnetic material protrusion, the unbalanced bias producing means producing a difference in the amplitude of the magnetic flux density between that applied to one element of the pair of magnetoelectric transducer elements and that applied to the other element so that the difference in the electric signal level corresponding to the difference in the amplitude of the magnetic flux density becomes greater than the hysteresis of the waveform shaping circuit.
In the magnetic object motion sensor of the invention, a difference occurs between the amplitudes of the magnetic flux applied to the respective magnetoelectric transducer elements and thus the magnetoelectric transducer elements are, in effect, unbalanced.
This allows a greater production tolerance and a greater dimensional tolerance. Furthermore, the moving magnet object sensor can operate with high accuracy even when the magnetic object moves at low speeds.
Preferably, the above unbalanced bias producing means includes a structure in which the line from the center of one element to the center of the other element of the pair of magnetoelectric transducer elements is slanted with respect to the direction of motion of the moving magnetic object.
The unbalanced bias producing means may also include a structure in which a side face of the magnet is slanted with respect to the direction of motion of the moving magnetic object.
The unbalanced bias producing means may also include a structure in which the magnet is magnetized in a direction slanted with respect to the normal to the surface of the moving magnetic object.
The unbalanced bias producing means may include a structure in which the position of the central axis of the pair of magnetoelectric transducer elements and the position of the central axis of the magnet are offset from each other.
Furthermore, the moving magnetic object may be a rotating magnetic object, and the unbalanced bias producing means may include a structure in which the position of the central axis of the pair of magnetoelectric transducer elements and the position of the central axis of the magnet are both offset from the position of the central axis in a radial direction of the rotating magnetic object.
According to another aspect of the present invention, there is provided a magnetic object motion sensor comprising: a pair of magnetoelectric transducer elements disposed opposite a magnetic material protrusion of a moving magnetic object; a magnet for generating bias magnetic flux toward the pair of magnetoelectric transducer elements; a pair of amplifiers for separately amplifying the electric signals output by the pair of magnetoelectric transducer elements; a differential amplifier for amplifying in a differential fashion the electric signals output via the pair of amplifiers; and a waveform shaping circuit for converting the differential signal output by the differential amplifier to a pulse signal corresponding to the edges of the magnetic material protrusion; the pair of magnetoelectric transducer elements being disposed in such a manner that one element is a predetermined distance apart from the other in the direction of motion of the moving magnetic object so that the pair of magnetoelectric transducer elements detect a change in magnetic flux caused by the motion of the magnetic material protrusion, the unbalanced bias producing means producing a difference in the amplitude of the magnetic flux density between that applied to one element of the pair of magnetoelectric transducer elements and that applied to the other element so that the difference in the electric signal level corresponding to the difference in the amplitude of the magnetic flux density becomes greater than the hysteresis of the waveform shaping circuit.
With this arrangement, a difference also occurs between the amplitudes of the magnetic flux applied to the respective magnetoelectric transducer elements and thus the sensitivity is, in effect, unbalanced between the magnetoelectric transducer elements.